You should always specify the :inverse_of option on AR associations

Directly from ActiveRecord documentation on bi-directional associations:

Bi-directional associations

When you specify an association there is usually an association on the associated model that specifies the same relationship in reverse. For example, with the following models:

class Dungeon < ActiveRecord::Base
  has_many :traps
  has_one :evil_wizard
end

class Trap < ActiveRecord::Base
  belongs_to :dungeon
end

class EvilWizard < ActiveRecord::Base
  belongs_to :dungeon
end

The traps association on Dungeon and the dungeon association on Trap are the inverse of each other and the inverse of the dungeon association on EvilWizard is the evil_wizard association on Dungeon (and vice-versa). By default, Active Record doesn’t know anything about these inverse relationships and so no object loading optimization is possible. For example:

d = Dungeon.first
t = d.traps.first
d.level == t.dungeon.level # => true
d.level = 10
d.level == t.dungeon.level # => false

The Dungeon instances d and t.dungeon in the above example refer to the same object data from the database, but are actually different in-memory copies of that data. **Specifying the :inverse_of option on associations lets you tell Active Record about inverse relationships and it will optimise object loading. For example, if we changed our model definitions to:

class Dungeon < ActiveRecord::Base
  has_many :traps, inverse_of: :dungeon
  has_one :evil_wizard, inverse_of: :dungeon
end

class Trap < ActiveRecord::Base
  belongs_to :dungeon, inverse_of: :traps
end

class EvilWizard < ActiveRecord::Base
  belongs_to :dungeon, inverse_of: :evil_wizard
end

Then, from our code snippet above, d and t.dungeon are actually the same in-memory instance and our final d.level == t.dungeon.level will return true.

There are limitations to :inverse_of support:

• does not work with :through associations.
• does not work with :polymorphic associations.
• for belongs_to associations has_many inverse associations are ignored.